1. Field of the Invention
The present invention relates to a field effect transistor using a compound semiconductor, and more particularly, to a gate electrode structure of HEMTs (High Electron Mobility Transistors), MESFETs (Metal Semiconductor Field Effect Transistors), and the like.
2. Description of the Related Art
Since a compound field effect transistor is especially used for high-frequency communication, low noise characteristics have been required therefore. FIG. 4 shows a schematic arrangement of a conventional compound field effect transistor. An insulating film 22 is formed on a semiconductor substrate 21 having an electrically isolated active region (not shown). Thereafter, drain and source electrodes 23 and 24 each acting as an ohmic contact and a gate electrode 25 acting as a Schottky gate are provided on the semiconductor substrate 21.
With the recent advance in the fine pattern structures of semiconductor devices, a demand has arisen for a compound semiconductor field effect transistor having a gate length of 0.25 .mu.m or 0.1 .mu.m. However, in the transistor shown in FIG. 4, it is difficult to obtain low noise characteristics because of an increase in the gate resistance of the gate electrode 25.
As shown in FIG. 5, therefore, even in the conventional transistor, the gate electrode 25 having a T-shaped structure in the cross-section has been proposed to increase the cross-sectional area, thereby suppressing an increase in the gate resistance. The characteristics of such a semiconductor device can be expressed by a high-frequency noise figure NF given by the following formula: EQU 1+2.pi.K.sub.F .times.f.times.Cgs.times.[(Rs+Rg)/g.sub.m ].sup.1/2
K.sub.F : fitting constant PA1 f: operating frequency PA1 Cgs: gate-source capacitance PA1 Rs: gate-source resistance PA1 Rg: gate resistance PA1 g.sub.m : transconductance
Assume that the length of an upper electrode portion 25' of the gate electrode 25, which is provided on the insulating film 22 at the side of the source electrode 24, is l; the width of its cross-sectional area, W; the gate length, Lg; the thickness of the insulating film 22, d; the vacuum dielectric constant, .epsilon..sub.0 ; and the dielectric constant of the insulating film 22, .epsilon., respectively. In this case, a capacitance increase .DELTA.C due to the formation of the upper electrode portion 25' of the gate electrode 25 on the insulating film 22 is given by .epsilon..sub.0 .times..epsilon..times.W.times.Lg/d. This parasitic capacitance is generated in parallel with the gate-source capacitance Cgs. Therefore, a gate-source capacitance Cgs' of the gate electrode 25 having the T-shaped cross-section becomes Cgs+.DELTA.C. Further, a gate resistance Rg' of the gate electrode 25 having the T-shaped cross-section becomes Rg'=(n.sup.-1).Rg, provided that the cross-sectional area of the gate electrode 25 is n times larger than that of the gate electrode 25 in FIG. 4.
In the transistor shown in FIG. 5, therefore, the high-frequency noise figure NF can be expressed as follows: EQU 1+2.pi.K.sub.F .times.f.times.(Cgs+.DELTA.C).times.[(Rs+(n.sup.-1).Rg)/g.sub.m ].sup.178
That is, the gate resistance Rg can be reduced by forming the gate electrode 25 as the T-shaped structure in the cross-section. However, since the gate electrode 25 extends to the insulating film 22, the gate-source capacitance Cgs is increased and the high-frequency noise figure NF cannot be effectively reduced.
As described above, the gate resistance can be reduced by forming the gate electrode as the T-shaped cross-section, corresponding to the gate length of the compound semiconductor field effect transistor. However, since the gate-source capacitance Cgs is increased, it is necessary to reduce the gate-source capacitance Cgs in order to obtain higher performance.